Maybe this error happens because unity tries to set the reference to the variable in the first frame, without having the reference information.
So, create a function to assign the FirebaseDatabase.DefaultInstance.RootReference after a short period of time. Like this:

void Start()
{
StartCoroutine(LateStart(1));
}

IEnumerator LateStart(float waitTime)
{
yield return new WaitForSeconds(waitTime);
dbReference = FirebaseDatabase.DefaultInstance.RootReference;
}

Hi , solved my problem with a simple thing, instead use a variable "DatabaseReference reference" and next equals FirebaseDatabase.DefaultInstance.RootReference, use directly in the code to save data like this - >

FirebaseDatabase.DefaultInstance.RootReference.Child("Users").Child("user1").Child("email").SetValueAsync(emailRegisterField.text.ToString()).ContinueWith(task =>
{
if (task.IsCompleted)
{
Debug.Log("SUCESSO");

}
else
{
Debug.Log("já foste");
}
}

If you are using a Default instance it's gonna struck even I experience.

So best way is instead of using default use
Firebasesatabase. Getinstance(app )

The app is the firebaseapp which we created at start by using and checking analytics.

This worked for me.

There is no solution because it's not a Unity issue. I've had this same problem and often the issue is that you have not properly closed the connection in firebase.

OnApplicationQuit()

- Any .ValueChanged subscriptions must be cleaned up
- call `FirebaseDatabase.DefaultInstance.App.Dispose();`

If you've properly cleaned up and disposed the firebase connection then the Unity Editor does not hang.

we're having the very same problem.
Please share more info as to what the solution is and why you cannot fix it?

Why won't fix?!